Serum throxine binding and capillary permeability affect the distribution of thyroxine in extrathyroidal tissues. The general purpose of the proposed studies is to describe the relationship between serum binding, capillary permeability and tissue thyroxine uptake. We are particularly interested in thyroxine delivery to tissues such as the uterus which, presumably because of low capillary permeability, are part of the slowly equilibrating thyroxine distribution space. Methods for determining serum iodothyronine binding will include measurement of free thyroxine and free triiodothyronine by ultrafiltration. Total triiodothyronine and possibly thyroxine determination will by immunoassay. As in previous studies capillary permeability will be determined by the rate of radioiodinated albumin transfer into the extracellular space. Tissue hormone uptake will be determined by tracer studies and immunoassay of nonisotopic thyroxine and triiodothyronine in tissue extracts. We also plan subcellular localization by radioautography. Studies are to include the effects of changes in serum binding capacity and tissue capillary permeability. Polyacrylamide gel electrophoresis and competitive binding studies will be applied to the question of serum protein binding specificity for triiodothyronine and thyroxine. Additional studies are proposed in which the nature of thyroxine binding will be investigated by spectrophotometric determination of complex formation with small molecules.